According to this post on Phil Sturgeon's blog (and the EllisLab folks) the current version of CodeIgniter 2.0 living in their Bitbucket repository is stable and has been for a while - despite some major changes.

CodeIgniter 2.0 is a major release, which means this has been a perfect chance for EllisLab to make major changes that effect compatibility. One example of a major change is that our Models now need to extend CI_Model instead of Model. That is an acceptable change for a major release, but think of the outcry if that was done after EllisLab had tagged v2.0.

He talks mentions the bugs currently in the tracker (quite a few) and how, despite those, the framework is still in a good, stable state. He also notes that EllisLab has to maintain both ExpressionEngine and MojoMotor on top of the framework so smaller patches and related functionality has just stemmed from that.

My advice to you, download the latest copy and have a play. If your application worked in v1.7.2 it will work in v2.0 and the upgrade is pretty simple.